Disk brakes are so excellent in stability at high speeds and high loads that they have recently been used not only for automobiles, but also for autobicycles or motorcycles.
It is necessary to heat-treat such a brake disk to a required hardness for brake feeling, wear-resistance, anticorrosion, and prevention of the metallic sound, i.e., the squeaking of the brake caused by friction at the time of braking. Together with such hardness control, the flatness and parallelism of the annular disk portion and hub fitting portion forming brake pad sliding surfaces are required to have great precision. The portion having holes, such as holes for fitting the brake disk and an axle inserting hole, is also required to have great precision.
There is a type in which the annular disk portion and hub fitting portion are separately shaped and then combined to be integral with each other, and a type in which both portions are integrally shaped from one sheet of material. Both types are selectively used depending on the size of the vehicle or the like. However, the latter type in which both portions are integrally shaped is very advantageous to manufacture and costs are minimized, but has the following problems in satisfying the above-mentioned conditions in its manufacture.
Generally, when a brake disk is adopted for an autobicycle, from the requirements in weight reduction and design, a plurality of window holes are made by punching or the like on the periphery of the truncated conical intermediate portion drawn and shaped between the annular disk portion and hub fitting portion. Also, on the hub fitting surface, a plurality of holes for fitting the disk to the hub are made, and a hole for inserting the axle or axle boss is made.
In the conventional shaping of such a brake disk, the integrally shaped brake disk is drawn and shaped with drawing dies while being pressed in the annular disk portion on its outer periphery with pressing dies. As the brake disk is drawn and shaped while being pressed on the annular disk portion at the time of shaping, the truncated conical intermediate portion between the hub fitting portion and this annular portion will be pulled in the peripheral direction to be thin and, at the same time, this pulling action will also influence the hub fitting portion. As a result, the hub fitting holes and axle inserting hole made in advance by punching or the like in the hub fitting portion will be distorted. The hub fitting surface will also deviate from its uniformity. Therefore, after the shaping, it is necessary to correct and mechanically work the holes and hub fitting surface. It is very difficult to thus mechanically work a shaped product made by being quenched simultaneously with the abovementioned shaping, and high in surface hardness. It is not desirable in workability and mass-productivity including the problems of the severability and tool life, and defeats the requirement of the reduction of cost of the brake disk. Not only in the case of forming the above-mentioned hub fitting portion in advance as in the above, but also in the case of shaping it by simultaneously punching and drawing it, the hub fitting surface will slip, the shaped holes will be deformed, and thus the same corrections mentioned above will be required. As a result of such corrections, the precision of fitting the brake disk will decrease, the pad and annular disk portion will be partially worn, the wear will be accelerated, or the brake performance will decrease.
Therefore, in a brake disk, it is very important that not only the precision in flatness and parallelism of the annular disk portion forming pad sliding surfaces, but also the precision of fitting the hub fitting portion, is kept high. It is desirable that the fitting holes shaped in the hub portion are kept at a high precision. In shaping the brake disk, it is more favorable to production and mass-production in addition to the above to require no correcting work.